This invention relates to a toroidal propeller having higher thrust per unit power along with a significant reduction in acoustic signature than is achievable with conventional propeller designs.
Multirotor drones rely entirely on the thrust generated by propellers both to stay aloft and to maneuver. The mechanical simplicity of these platforms drives their adoption in the commercial sector for a range of applications—cinematography, industrial inspection, airborne monitoring, aerial delivery, and even air taxis. However, recent psychoacoustic experiments conducted by NASA Langley on human test subjects shows that while overall annoyance to noise increases with sound level, small multirotor drones generate a high annoyance level at a given sound level than do road vehicles. See FIG. 1 that compares a time integrated A-weighted sound energy metric to perceived annoyance.
The primary noise sources for these multirotor drones are brushless DC motors and the aerodynamic noise generated by the propellers driven by those motors. Public acceptance of multirotor drones is critical to their widespread use; acoustically quieter propeller designs that are able to serve as drop-in replacements for conventional propellers may, more than any other technological advancement, accelerate the acceptance and wide use of drones in public spaces.
Closed form or box wings have been investigated in the past as a method to reduce losses associated with tip vortex generation while also enhancing the structural integrity of the wing. Investigators at the Massachusetts Institute of Technology Lincoln Laboratory have studied the aerodynamic feasibility of so-called ring wings, wherein the tips of two lifting surfaces are linked via a lofted surface to generate a non-planar closed structure. This research demonstrated lift-to-drag ratio improvements of a ring wing relative to a comparable planar wing of 40-60% as shown in FIGS. 2a and 2b derived from interactions between lifting surfaces and a significant reduction in the strength of the trailing tip vortex, a key source of aerodynamic noise.
With reference now to FIGS. 3a, 3b and 3c, example prior art propellers are shown. FIG. 3a is a design disclosed in U.S. Pat. No. 4,445,817. FIG. 3b is a shrouded propeller design from U.S. Pat. No. 5,096,382. FIG. 3c, described in U.S. Pat. No. 6,736,600, is somewhat similar to the design disclosed in this patent application. The design in FIG. 3c utilizes a split propeller design wherein each blade is split at an arbitrary distance from the main propeller hub and extended forward and aft with respect to the propeller rotation direction.